The Institute of Electrical and Electronics Engineers (IEEE) has proposed a series of standards for the field of wireless access technology applications, such as IEEE 802.11 for wireless local area networks and IEEE 802.15 for implementing short-range interconnections based on the Bluetooth specification. Currently, IEEE 802.16 being developed is a standard for air interfaces in IEEE wireless metropolitan area networks, and as a core technology of broadband wireless accesses it is attracting more attention. Especially, the IEEE 802.16j specification concerning multi-hop relays are attracting lots of attention from industry.
A plurality of features will be added to an original wireless access network, such as a synchronization operation among a Base Station (BS), a Relay Station (RS) and a Mobile Station (MS), since the IEEE 802.16j is intended to introduce RSs to relay signals between BSs and MSs. To cope with this situation, it is necessary to reconstruct corresponding frame structures so as to support multi-hop relays, thereby enhancing coverage, throughput and system capacity. Meanwhile, it is preferred that the constructed frames are transparent to MSs.
A lot of frame constructing methods based on the IEEE 802.16j frame have been developed. An example is a frame constructing method developed by Motorola Corporation, and the frame structure being constructed according to the method is as shown in FIG. 1. With the solution, it is needed to determine the RSs and MSs in direct communication with a BS firstly, which are called “direct-communication stations”. Preamble 1 and broadcast information associated with these “direct-communication stations” and data sent to these direct-communication stations are set in a DL sub-frame 101 as a field 1011. Thereafter, it is needed to determine MSs communicating with the BS via RSs, which are called “relay-MS”, and Preamble 2 and broadcast information associated with these “relay-MS” and DL data are set in the DL sub-frame 101 as another field 1012. For a UL sub-frame 102, it is needed to sequentially determine MSs and RSs which send data to the BSs. Then, respective preamble and UL data of a MS are set in the UL sub-frame 102 as fields 1021, 1022 and 1023 in the sequence of a MS communicating with the BS directly, then a MS communicating with a RS, and then a RS. Finally, the DL sub-frame 101 and UL sub-frame 102 are combined into a completed frame. According to the solution, the communication between a RS and a MS is kept through maintaining the specific field 1012 in the frame.
It can be seen from FIG. 1 that with respect to the preamble for implementing synchronization, the preamble 1 of the MS communicating with the BS directly is different from the preamble 2 of the MS communicating with the RS. Thus, a complex switching process would be performed when a MS moves between a RS's coverage domain and a BS coverage domain. As a result, this solution is not transparent enough for MSs.
In addition, with the solution, a BS and a RS send preambles at different time, respectively, frustrating the MS's synchronization operation.
Another frame constructing method is provided by Huawei company, and the frame structure being constructed according to the method is as shown in FIG. 2. Likewise, it is needed to determine RSs and MSs in direct communication with a BS firstly according to the method. What is different is that preamble A, broadcast information, DL data and UL data which are associated with these “direct-communication station” are then set in a sub-frame A together according to the method. Then, it is needed to determine MSs communicating with RSs, and preamble B, DL data and UL data which are associated with these “relay-MSs” are set in a sub-frame B, following the sub-frame A. The sub-frame A in the joint-frame constructed according to the method is in charge of relaying between BSs and RSs, while the sub-frame B takes charge of cell coverage. In the solution, there is included in each sub-frame (sub-frame A or B) both DL data and UL data. The communication between a BS and a RS is implemented through transfer of UL and DL data of which positions are fixed in the sub-frame.
It can be seen from fixed frame structure shown in FIG. 2 that transfers of UL and DL data between a BS and a RS are processed first, and then transfers of UL and DL data between the RS and a MS are processed. Thus, in the case of the UL communication of data from the MS to the BS via the RS, the MS is unable to send UL data (processed with the sub-frame B) to the RS when the data between the BS and the RS is being processed (by using the sub-frame A), and thus the data of MS can not be transferred in time, resulting in long delays. Frame structures of RSs #1 and #2 correspond to the frame structure of the BS are shown in FIG. 2, and their respective frame header is synchronized with the frame of the BS, and further it is necessary to obtain MAP information from the BS.
Moreover, this solution requires extra frequency planning and complex handover operation when a MS moves, if BSs with a RS are deployed adjacent to general BSs. This is because that the joint frame may not be aligned with a general frame, which will cause interference. This solution may also result in MS resynchronization when crossing different cells.